Streptococcal IgA-binding proteins bind in the Calpha 2-Calpha 3 interdomain region and inhibit binding of IgA to human CD89.

Certain pathogenic bacteria express surface proteins that bind to the Fc part of human IgA or IgG. These bacterial proteins are important as immunochemical tools and model systems, but their biological function is still unclear. Here, we describe studies of three streptococcal proteins that bind IgA: the Sir22 and Arp4 proteins of Streptococcus pyogenes and the unrelated beta protein of group B streptococcus. Analysis of IgA domain swap and point mutants indicated that two loops at the Calpha2/Calpha3 domain interface are critical for binding of the streptococcal proteins. This region is also used in binding the human IgA receptor CD89, an important mediator of IgA effector function. In agreement with this finding, the three IgA-binding proteins and a 50-residue IgA-binding peptide derived from Sir22 blocked the ability of IgA to bind CD89. Further, the Arp4 protein inhibited the ability of IgA to trigger a neutrophil respiratory burst via CD89. Thus, we have identified residues on IgA-Fc that play a key role in binding of different streptococcal IgA-binding proteins, and we have identified a mechanism by which a bacterial IgA-binding protein may interfere with IgA effector function.

Cross-protection between group A and group B streptococci due to cross-reacting surface proteins.

The R28 protein of group A streptococcus (GAS) and the Rib protein of group B streptococcus (GBS) are surface molecules that elicit protective immunity to experimental infection. These proteins are members of the same family and cross-react immunologically. In spite of extensive amino acid residue identity, the cross-reactivity between R28 and Rib was found to be limited, as shown by analysis with highly purified proteins and specific antisera. Nevertheless, immunization of mice with purified R28 conferred protection against lethal infection with Rib-expressing GBS strains, and immunization with Rib conferred protection against R28-expressing GAS. Thus, R28 and Rib elicited cross-protective immunity. Characterization of many clinical GAS and GBS isolates expressing R28 or Rib, respectively, indicated that most of them expressed proteins similar to those of the reference strains. Analysis of these data suggests that cross-protection may influence the outcome of natural infections with R28-expressing GAS and Rib-expressing GBS.

Group B streptococcal surface proteins as targets for protective antibodies: identification of two novel proteins in strains of serotype V.

Strains of group B streptococcus (GBS) express surface proteins that confer protective immunity. In particular, most strains of the four classical capsular serotypes (Ia, Ib, II, and III) express either of the Rib and alpha proteins, two members of the same protein family. Here, we report a study of surface proteins expressed by strains of serotype V, which has recently emerged as an important serotype among GBS strains causing serious disease. Two novel GBS proteins were identified, purified, and characterized. One of these proteins, designated Fbs, was immunologically unrelated to other GBS surface proteins. This approximately 110-kDa protein was found in 15 of 49 (31%) type V isolates but in few strains of other serotypes. The Fbs proteins expressed by different strains showed limited variation in size. The most common surface protein among type V strains, found in 29 of 49 (59%) isolates, was designated Rib-like, since it cross-reacted with Rib but was not immunologically identical to Rib. Characterization of this Rib-like protein showed that the N-terminal sequence (12 residues) was identical to that of alpha, although these two proteins lacked cross-reactivity. The biochemical and immunological properties of the Rib-like GBS protein indicate that it is closely related to the R28 protein of Streptococcus pyogenes. Importantly, passive and active immunization experiments with mice showed that the Fbs and Rib-like proteins are targets for protective antibodies. These two proteins are therefore of interest for analysis of pathogenic mechanisms and for vaccine development.

The R28 protein of Streptococcus pyogenes is related to several group B streptococcal surface proteins, confers protective immunity and promotes binding to human epithelial cells.

The R28 protein is a surface molecule expressed by some strains of Streptococcus pyogenes (group A streptococcus). Here, we present evidence that R28 may play an important role in virulence. Sequence analysis demonstrated that R28 has an extremely repetitive sequence and can be viewed as a chimera derived from the three surface proteins Rib, alpha and beta of the group B streptococcus (GBS). Thus, the gene encoding R28 may have originated in GBS. The R28 protein promotes adhesion to human epithelial cells, as shown by experiments with an R28-negative mutant and by the demonstration that antibodies to highly purified R28 inhibited adhesion. In a mouse model of lethal intraperitoneal S. pyogenes infection, antibodies to R28 conferred protective immunity. However, the virulence of an R28-negative mutant was similar to that of the parental strain in the intraperitoneal infection model. Together, these data indicate that R28 represents a novel type of adhesin expressed by S. pyogenes and that R28 may also act as a target for protective antibodies at later stages of an infection. We consider the hypothesis that R28 played a pathogenetic role in the well-known epidemics of childbed fever (puerperal fever), which were caused by S. pyogenes. A role for R28 in these epidemics is suggested by epidemiological data.

An IgA-binding peptide derived from a streptococcal surface protein.

Surface proteins that bind to the Fc part of human IgA are expressed by many strains of Streptococcus pyogenes, a major human pathogen. Studies of these proteins have been complicated by their size and by their ability to bind human plasma proteins other than IgA. Here, we describe a synthetic 50-residue peptide, derived from streptococcal protein Sir22, that binds human IgA but not any of the other plasma proteins known to bind to Sir22. The peptide binds serum IgA and secretory IgA and binds IgA of both subclasses. Evidence is presented that the peptide folds correctly both in solution and when it is immobilized and that it readily renatures after denaturation. Together, these data indicate that the peptide corresponds to a protein domain that binds IgA with high specificity. This is the first report of an IgA-binding domain that retains its properties in isolated form.